The first X-Men movie came out in 2000, long before all of our movie trailers started showing up on YouTube and clever marketing ploys could be forwarded with a click of the mouse. So the marketing department for X-Men decided to try something a bit silly: they played a joke on the people who open Congressional mail.
Below is an account of my personal experience interning for a Congressman in the spring of 2000, just a few months before X-Men was released. As goofy as it was, this remains my single favorite marketing strategy for a movie ever:
Senator Kelly's website, MutantWatch.com is no longer online, but you can still see it on the Wayback Machine. Confession: I don't have the actual letter that I received on that fateful day, so I lifted the text from the website. The tone and content of the letter was, if not the same, at least very similar.
Also, I've spotted some of the Senator Kelly campaign buttons and bumper stickers on eBay, in case you're curious as to what they actually look like. Sadly, I was not able to track down the video that went with them. If anyone still has it (or has seen it online), I'd be tickled to see it again.